Optical harmonic generator

ABSTRACT

An harmonic generator system for generating at least two output beams (6a, 6b, 6c) of higher order harmonic radiation from an input beam (1) of radiation of frequency ω comprises at least two non linear optical crystals (3a, 3b, 3c) arranged in series. Radiation output from each crystal stage has first and higher order harmonic components. Chromatic separators (5a, 5b, 5c) separate beams of radiation output from each crystal and selected beams are then passed through a telescope (9a, 9b) to the next crystal in the series, thereby generating further radiation beams of first and higher order harmonics. Multiple beams of second, 2ω, third 3ω, or fourth, 4ω, harmonic radiation may be output from the system and may be used to pump various stages of a secondary laser system or an optical parametric oscillator. Alternatively, the output beams of higher order harmonic radiation may be coherently combined to form a single output. The system is capable of yielding considerably high conversion efficiencies, approaching 100%. The system for generating multiple beams of second harmonic radiation preferably comprises a three non linear crystals, for example potassium titanyl phosphate (KTP) or potassium dihydrogen phosphate (KDP), such that three beams of second harmonic radiation are generated. The telescope magnofications may be variable so that the relative intensity of radiation output at each crystal stage may be varied.

The invention relates to an harmonic generator system for efficientlygenerating higher order harmonic radiation from an input beam offundamental frequency radiation. In particular, the system may be usedto generate beams of second, third or fourth harmonic radiation. Thesystem is particularly suitable for use with lasers which are used topump a secondary laser system as multiple output beams may be generated,each being used to drive separate stages of the laser system.

Conventionally, the generation of second harmonic frequencies, 2ω, maybe achieved by irradiating an appropriate crystal with a beam of primaryradiation of frequency ω. Some fraction of the incident energy isconverted into the second harmonic within the crystal. For example,crystals which may be used for the generation of second harmonicfrequencies are potassium dihydrogen phosphate (KDP) and ammoniumdihydrogen phosphate (ADP).

Second harmonics of a primary laser beam are often used to pump asecondary laser system, for example a dye laser. It is common in mostsecondary lasers of this type to sub-divide the pumping beam (the secondharmonic) into a series of lower energy beams, graded in intensity,which are then used to pump the various stages of the secondary lasersystem such as the oscillators and the amplifiers.

The efficiency of the second harmonic generation process depends on thecrystal material used and the power of the incident primary radiation.Typically, the efficiency with which second harmonics can be generatedusing a conventional frequency doubling crystal is only 50% to 60% ofthe primary laser source. The second harmonic may be separated using aprism arrangement or a dichoric mirror and the remaining fraction offundamental radiation is wasted. Because of the limitation of theefficiency with which higher frequencies can be produced, powerful andbulky lasers may be needed in order to achieve sufficiently energeticsecond harmonic frequency beams for pumping applications.

The present invention relates to a system for generating a plurality ofbeams of multiple order harmonic radiation from an input beam offundamental frequency radiation with a conversion efficiency approaching100%. In particular, the system may be used to generate a plurality ofbeams of second, third or fourth harmonic radiation.

The system is particularly suitable for use with lasers which are usedas pump sources for a secondary laser system as the output beams areconveniently split into separate beams which may be used to pump theseparate stages of the secondary laser system. The system thereforeenables a smaller and more convenient primary laser to be used toachieve sufficiently energetic beams of multiple order harmonicradiation than can be achieved using known techniques.

According to the present invention, a system for generating at least twooutput beams of higher order harmonic radiation from an input beam offundamental radiation comprises;

a first sample of non linear optical material for receiving the inputbeam and generating output radiation comprising beams of fundamental andhigher order harmonic radiation,

means for separating the beams of fundamental radiation and higherharmonic radiation output from the first sample,

at least one further sample of non linear optical material, wherein thefirst and further samples are arranged in series, each further samplehaving, associated means for increasing the intensity of radiationincident on the further sample and means for separating beams offundamental and higher order harmonic radiation output from the furthersample,

wherein selected beams of radiation output from each of the first andfurther samples pass through the subsequent sample in the series andgenerate further beams of fundamental and higher order harmonicradiation.

In one embodiment of the invention, the first and further samples arearranged in series such that the beams of fundamental radiation outputfrom each of the first and further samples pass through the next samplein the series, such that at least two output beams of second harmonicradiation may be generated. Preferably, the system comprises a firstsample of non linear optical material and two further samples of nonlinear optical material, such that three output beams of second harmonicradiation may be generated.

The system may also comprise means for supplying the input beam offundamental radiation. For example, the input beam of fundamentalradiation may be supplied by a laser.

In another embodiment of the invention the system may also include asub-system for deriving the input beam of radiation from a beam ofprimary radiation of frequency ω, wherein said sub-system comprises;

an additional sample of non linear optical material for receiving thebeam of primary radiation and generating output radiation comprisingprimary radiation, ω, and second harmonic radiation 2ω,

means for separating the beams of primary and second harmonic radiationoutput from the additional sample and

means for increasing the intensity of radiation incident on the firstsample,

whereby the beam of second harmonic radiation output from the sub-systemis input to the system, such that that system may be used to generate atleast two output beams of fourth harmonic radiation.

In this embodiment of the invention, the system preferably comprises afirst sample of non linear optical material and two further samples ofnon linear optical material such that three output beams of fourthharmonic radiation may be generated.

The system may also comprise means for supplying primary radiation tothe sub-system, for example a laser.

The means for increasing the intensity of radiation incident on any ofthe first or further samples may be a refracting telescope, a system ofreflecting telescopes or a system of anamorphic prisms.

The means for increasing the intensity of radiation incident on any ofthe first or further samples may have a variable magnification such thatthe relative intensities of the output beams of higher order harmonicradiation generated by the system may be varied by varying themagnification.

The means for separating fundamental radiation and higher harmonicradiation output from any of the first or further samples may bechromatic separators, for example dichroic mirrors, prisms orpolarisers.

In another embodiment of the invention the system may also include asub-system for deriving the input beam of radiation from a beam ofprimary radiation of frequency ω, wherein said sub-system comprises;

an additional sample of non linear optical material for receiving thebeam of primary radiation and generating output radiation comprisingprimary radiation, ω, and second harmonic radiation, 2ω,

whereby the beam of radiation output from the sub-system is input to thesystem such that the system may be used to generate at least two outputbeams of third harmonic radiation.

In this embodiment of the invention the system preferably comprises afirst sample of non linear optical material two further samples of nonlinear optical material, such that three output beams of third harmonicradiation may be generated.

The system may also comprise means for providing primary radiation, forexample a laser.

The additional, first or further samples of non linear optical materialmay be non linear optical crystals. For example, potassium titanylphosphate (KTP), potassium dihydrogen phosphate (KDP), deuterated KDP(KD*P), caesium dihydrogen arsenate (CDA), deuterated CDA (CD*A), betabarium borate (BBO) or lithium triborate (LBO) are suitable-crystals.

The output beams of higher order harmonic radiation may be used to driveseparate stages of a secondary laser system. Alternatively, the beamsmay be combined coherently to form a single output beam. For example,the means for coherently combining the output beams of higher harmonicradiation may comprise a stimulated Brillouin scattering (SBS) cell andat least one Brillouin amplifier.

The invention will now be described, by example only, with reference tothe following figures in which;

FIG. 1 shows a diagram of a conventional system which may be used toproduce frequency doubled radiation from a primary laser and which isthen used to pump a secondary laser.

FIG. 2 shows a diagram of an improved efficiency system which may beused to produce three beams of frequency doubled radiation from aprimary laser.

FIG. 3 shows the spatial profiles of second harmonic generated beams ateach of three crystal stages (as shown in FIG. 2) for (a) 60 MW/cm² and(b) 90 MW/cm² beams.

FIG. 4 shows the temporal profiles of second harmonic generated beams ateach of three crystal stages (as shown in FIG. 2) for (a) 60 MW/cm² and(b) 90 MW/cm² beams.

FIG. 5 shows a diagram of a system of anamorphic prisms which may beused to increase the intensity of fundamental frequency radiationincident on each crystal.

FIG. 6 shows a diagram of a system which may be used to coherentlycombine several second harmonic radiation beams outputs from the systemshown in FIG. 2.

FIG. 7 shows how the system of the present invention may be used togenerate output beams of fourth harmonic radiation from a fundamentalfrequency beam and

FIG. 8 shows how the system of the present invention may be used togenerate output beams of third harmonic radiation from a fundamentalfrequency beam.

Referring to FIG. 1, an incident beam of radiation 1, with frequency ω,is emitted from a primary laser 2 and is passed through a frequencydoubling crystal 3. As a result, some fraction of the initialfundamental energy is converted into the second harmonic of frequency2ω. The radiation 4 exiting the crystal 3 therefore comprises componentsof radiation with frequencies of ω and 2ω. The frequencies may then beseparated by means of a dichroic separator 5, for example a dichroicmirror, or any other chromatic separator, such as a prism. The secondharmonic beam 6 may then be used to pump a secondary laser 7. Theconversion efficiency of this system would typically be 50% to 60%.

An improved second harmonic efficiency may be achieved by using thesystem shown in FIG. 2. The primary laser 2 emits a beam of fundamentalradiation 1 with frequency ω. The radiation 1 is then passed through aseries of frequency doubling crystals 3a,3b,3c, dichroic mirrors5a,5b,5c and telescopes 9a,9b. The crystals may be any non linearoptical crystal capable of generating second harmonic frequencies ofincident fundamental frequency radiation. For example, suitable chi(2)materials which may be used are potassium titanyl phosphate (KTP),potassium dihydrogen phosphate (KDP), deuterated KDP (KD*P), caesiumdihydrogen arsenate (CDA), deuterated CDA (CD*A), beta barium borate(BBO) and lithium triborate (LBO). The crystals used may all be of thesame material or various different crystals may be used. Typically thecrystals have a pathlength of 10 mm.

On passing through the first crystal 3a, the primary radiation offrequency ω gives rise to radiation having second harmonic frequencycomponents. The beam of radiation 4a transmitted by the crystal 3atherefore contains frequency components ω and 2ω. The dichroic mirror 5aseparates the frequencies so that the fundamental frequency beam 8a ispassed to a refracting telescope 9a while the frequency doubled beam 6amay be passed to a secondary laser system (not shown) for pumpingpurposes. Again, a polariser or a prism may be used instead of thedichroic mirror 5a to separate the beams of fundamental and secondharmonic frequency radiation.

On passing through the telescope 9a, the diameter of the fundamentalfrequency beam 8a is reduced so that the intensity is high enough onentering the second crystal 3b to allow efficient conversion. Thedemagnifying power of the telescope is usually chosen to restore thepeak intensity to a similar level as the primary fundamental frequencybeam 1. By making the magnification of each telescope variable, therelative intensity of second harmonic radiation generated at eachcrystal stage may be varied. A telescope may also be mounted in front ofthe first crystal 3a to increase the intensity of fundamental frequencyradiation entering the first crystal, although this is not essential.

On passing through the second crystal 3b, the fundamental frequency beam8a gives rise to the generation of second harmonic frequencies. Asbefore, a dichroic mirror 5b may be used to separate the fundamentalfrequency beam 8b and the frequency doubled beam 6b. The frequencydoubled beam 6b may then be used to pump a secondary laser (not shown)while the fundamental frequency beam 8b is passed through a thirdcrystal 3c and the process is repeated.

The conversion efficiency obtained using the system shown in FIG. 2depends on the particular crystals used, the number of crystal stages inthe system and the telescope demagnifications. By using a combination ofthree KTP crystals, each separated by a telescope and a dichroic mirror,a second harmonic conversion efficiency of greater than 97% may beachieved. Additional crystals, telescopes and dichroic mirrors may beincluded to increase the efficiency further, with diminishing return asthe residual fundamental energy is used up. However, since the residualfundamental energy is telescoped down at each crystal, the requiredcrystal size becomes increasingly smaller at each stage.

Using the system shown in FIG. 2 with a 10 Hz, injection seeded Nd:YAGlaser and three KTP crystals, a total conversion efficiency from 1.06 μmfundamental radiation to 532 nm radiation of greater than 97% may beobtained. In this case, the initial 1.06 μm energy was 207 mJ and thetriple conversion resulted in a total of 201 mJ of 532 nm laserradiation and compares to a single KTP crystal conversion ofapproximately 65%.

Adding three crystals in series without dichoric mirrors and telescopesdoes not increase the efficiency notably beyond that of a single crystaland under certain conditions may even reduce the second harmonic output.This is partly due to phase matching requirements and the reducedintensity of the fundamental frequency beam as the second harmonic beamis generated.

The multiple beams generated using the second harmonic generator systemshown in FIG. 2 are particularly useful for pumping secondary lasers andsimilar devices as the total harmonic radiation is convenientlyseparated into beams of graded energy. In conventional systems, it isusual to separate a single second harmonic pump laser beam into severalfractions to separately pump oscillator, preamplifier and poweramplifier stages of the secondary laser system. The need for a series ofbeam splitters to divide up a single beam is therefore eliminated in thepresent system.

The last second harmonic beam generated 6c will be the weakest and istherefore ideally suited to pump a laser oscillator or an opticalparametric oscillator. The penultimate beam 6b will be of higher energyand may therefore be used to drive a laser preamplifier or opticalparametric preamplifier. The first second harmonic beam 6a generated hasthe greatest energy and is therefore ideally suited for pumping a poweramplifier stage.

FIG. 3 shows the theoretical three stage spatial profiles of generatedsecond harmonic beams for input beams of 60 MW/cm² (FIG. 3(a)) and 90MW/cm² (FIGS. 3(b)). FIG. 4 shows the three stage temporal profiles ofgenerated second harmonic beams for input beams of 60 MW/cm² (FIG. 4(a))and 90 MW/cm² (FIGS. 4(b)). A useful feature of the system is that theduration of the second harmonic pulses that are generated 6a,6b,6cprogressively lengthen and the spatial profiles of the pulses can bemade more flat-topped with each successive crystal stage. This is due tothe effects of pump depletion. This feature makes the systemparticularly suitable to provide a pump source for optical parametricoscillators and other laser systems as the flat topped spatial profilecan reduce the risk of damage and the corresponding longer pumping pulsecan help to provide a better temporal overlap between the oscillatoroutput pulse and the subsequent amplifier pump pulses.

The conversion efficiency associated with each successive stage alsobecomes progressively higher due to the flattening of the temporal andspatial profiles. For example, the theoretical conversion efficienciesfor three crystal stages (in sequential order) for an initial Gaussian60 MW/cm² peak input intensity are 52%, 75% and 81%, giving a totalconversion of nearly 98%. the increased conversion efficiencies at thesecond and third stages are due to the flattening of the spatialprofile.

Almost complete conversion into the second harmonic is possible withhigher initial pumping intensities. For example, 99.6% conversion may beachieved for a 90 MW/cm² Nd:YAG 1.06 μm beam, but the relatively smallgain in overall efficiency can have a detrimental effect For such stronginitial pumping intensities the output spatial and temporal profiles canbecome distorted, particularly at the third stage, due to the high pumpdepletion. In practice, however, for most laboratory sources withGaussian beams, it becomes difficult to employ telescopes with the idealdemagnifications necessary to cope with conversion efficiencies much inexcess of 50% for the first crystal stage. The reduced beam diameters,particularly at the third stage, and the physical length of thetelescopes make it difficult to remain in the near field and preserveintensity over realistic propagation distances.

Relay imaging between the crystal stages could be used to allow nearfield conversion at any demagnification but vacuum telescopes would beneeded to avoid air breakdown at the intermediate foci. Less bulkyGalilean telescopes are a more practical solution for moderatedemagnifications.

It is not necessary, however, to precisely reproduce peak intensities ateach stage in order to achieve a high overall conversion efficiency. Theexact spatial and temporal profiles achieved at each crystal stage canbe controlled by an appropriate choice of the preceding telescope,within the limitations of the required overall conversion efficiency.For example, it is possible to achieve a high conversion efficiency forthe first crystal stage and then obtain a more moderate conversion (thanwould be possible for a fully restored peak intensity) at each of thesubsequent two stages by using lower demagnification telescopes. Eventhough strong pump depletion occurs at the first stage, the moderatedconversion efficiencies for the subsequent harmonic generators can allowgood quality beams to be generated at all stages.

Furthermore, the crystals 3a,3b3c may be mounted such that the angle atwhich incident radiation enters each one may be varied. By varying thisangle the amount of second harmonic radiation generated at each state(6a,6b,6c) may be varied, although in most cases the maximum possibleoutput is likely to be required.

Referring to FIG. 5, a system of anamorphic prisms 10 may be usedinstead of the refracting telescopes 9a,9b (see FIG. 2) in order toincrease the intensity of radiation incident on each of the crystals3a,3b,3c. Alternatively, a system of reflecting telescopes may be usedor a system axis parabolic mirrors.

Referring to FIG. 6, the second harmonic beams 6a, 6b, 6c may also becoherently combined to form a single output 11. A stimulated Brillouinscattering (SBS) cell 15 imparts a small frequency downshift to thesecond harmonic signal 6c and generates an output signal 17. Thefrequency downshift is equivalent to the acoustic frequencycharacteristic of the Brillouin material. Suitable materials for use inthe SBS cell would be liquid N-hexane, liquid N-pentane or gaseous C₂F₆.

In this configuration, the second harmonic signal 6c is used as theinput signal to a Brillouin amplifier 12a and one of the other secondharmonic signals 6b is used to pump this amplifier. The amplified outputfrom this Brillouin amplifier 12a becomes the signal for a secondBrillouin amplifier 12b, and so forth depending on the number of secondharmonic outputs from the system.

Porsers 13 and quarter wave plates 14 are used to direct the pump beamsinto the Brillouin amplifiers and to convert linearly polarisedradiation into circularly polarised radiation with the polarisers 13 andquarter wave plates 14 arranged such that beams 6a and 6b do not reachthe SBS cell 15. As the signal 17 proceeds through the Brillouinamplifiers, energy in beams 6a and 6b is imparted to 6c, resulting in asingle combined output beam 11.

In principle, amplification can proceed in either direction, startingwith either the weakest or the strongest beam, but since the SBS cellefficiency is never 100% efficient it is preferable to derive theamplifier signal from the weakest beam (i.e. 6c) within the thresholdlimitations of the SBS cell.

Mode matching optics, for example telescopes (not shown), are alsoincluded between the Brillouin amplifiers 12a, 12b as the various beamsare of different initial diameters, and to reach the required gIL value(Brillouin gain) for the efficient operation of the Brillouinamplifiers. Optical delay lines (not shown) are also included in beams6a,6b,6c to ensure proper temporal overlap of the various beams in theBrillouin amplifiers 12a,12b.

Using the system shown in FIG. 6, an intrinsic SBS conversion efficiencyof up to 95% may be achieved, giving an total frequency doublingefficiency of 97%×95%=92%. This is a considerable improvement on thefrequency doubling efficiencies typically obtained with a single KTPcrystal (typically 60%).

Referring to FIG. 7, the principle behind the system may also be used togenerate multiple beams of radiation with four times the frequency ofthe fundamental radiation, 4ω (fourth harmonic generation). In this casean additional frequency doubling crystal 18 is used and the output beam4a from this crystal will have fundamental and second harmonic frequencycomponents which may then be separated by a chromatic separator 5a. Thecrystals for use in this system may be any of one the examples mentionedpreviously.

The separated second harmonic beam 6a then becomes the input beam to athree crystal system shown in FIG. 2 (i.e. the beam input to the firstof the three crystals has frequency 2ω), with the addition of a thirdtelescope 19 mounted in front of the first of the three crystals 3a. Thesystem therefore generates three output beams 20a,20b,20c of four timesthe frequency of the fundamental beam (and two times the frequency ofbeam 6a). As described previously for second harmonic beams the fourthharmonic output beams 20a,20b,20c may be used separately to driveindividual stages of an amplifier system or may be combined coherentlyto form a single output. Referring to FIG. 8, the principle behind thesystem may be used to generate multiple beams of radiation with threetimes the frequency of the fundamental radiation from the laser source(third harmonic generation). Fundamental frequency radiation 1 from thelaser 2 enters a first second harmonic crystal 3a. This crystal ischosen and adjusted so that approximately 67% conversion efficiency intothe second harmonic is achieved. At this conversion efficiency, thefundamental harmonic photons and the second harmonic photons are matchedin numbers.

The conversion process for generating third harmonic radiation involvesadding together one photon having fundamental frequency, ω, to onephoton having second harmonic frequency, 2ω, therefore generating aphoton with third harmonic frequency, 3ω, which is equal in energy tothe sum of the first and second harmonic photons. The radiation 4aoutput from the first crystal 3a is then passed through a second crystal3b so as to generate third harmonic radiation. Assuming that theconversion efficiency is less than 100%, radiation 21a emerging from thesecond crystal 3b will contain radiation having frequency components ω,2ω and 3ω. A first dichroic separator 5a is then used to split off thethird harmonic radiation 22a.

After passing through the first dichroic separator 5a, the residualradiation beam 23a containing fundamental and second harmonic frequencycomponents is passed through a telescope 9a to reduce the beam size.This increases the intensity of radiation 23a and ensures that theconversion efficiency is high when the radiation passes through thesubsequent crystal in the chain, crystal 3c.

The process is then repeated; a second dichroic separator 5b separatesthird harmonic radiation 22b generated in the crystal 3c and residualradiation 23b (having frequency components ω and 2ω), residual radiationis passed through a second telescope 9b and enters a third crystal 3d,generating a third output beam of third harmonic radiation 22c.

The crystals for use in the system of FIG. 8 may be any of one theexamples mentioned previously. The beam transmitted by each of thedichroic separators 5a,5b,5c contain two frequency components, ω and 2ω,and it is important that these two wavelengths remain together in asingle beam prior to entering the next telescope in the sequence. Thedichroic separators and the telescopes must therefore be free fromchromatic aberrations and prisms may not be used in this particularembodiment of the invention. Polarisers, however, could be used.

Likewise, the telescopes 9a,9b,9c must be achromatic and cannot bereplaced by anamorphic prisms (as described previously for second andfourth harmonic generation systems). Mirror telescopes would be suitablefor use in the third harmonic generation system.

If the numbers of ω and 2ω photons are not balanced after the firstcrystal stage, the conversion into the second harmonic is not maximisedand the conversion efficiency at each of the subsequent stages isreduced. The system would therefore generate third harmonic radiationwith a lower conversion efficiency.

I claim:
 1. A system for generating an output beam of higher orderharmonic radiation from an input beam of fundamental radiationcomprising;a first sample of non linear optical material for receivingthe input beam and generating output radiation comprising beams offundamental and higher order harmonic radiation, means for separatingthe beams of fundamental radiation and higher harmonic radiation outputfrom the first sample, at least one further sample of non linear opticalmaterial, wherein the first and further samples are arranged in series,each further sample having associated means for increasing the intensityof radiation incident on the further sample and means for separatingbeams of fundamental and higher order harmonic radiation output from thefurther sample, wherein selected beams of radiation output from each ofthe first and further samples pass through the subsequent sample in theseries and generate further beams of fundamental and higher orderharmonic radiation, characterised in that the apparatus furthercomprises a stimulated Brillouin scattering (SBS) cell and at least oneBrillouin amplifier for coherently combining the output beams of higherorder harmonic radiation.
 2. The system of claim 1, the first andfurther samples generating at least two output beams of second harmonicradiation, the two or more output beams of second harmonic radiationbeing coherently combined by means of the stimulated Brillouinscattering (SBS) cell and the one or more Brillouin amplifier.
 3. Thesystem of claim 2 comprising a first sample of non linear opticalmaterial and two further samples of non linear optical material forgenerating three output beams of second harmonic radiation, the threeoutput beams of second harmonic radiation being coherently combined bymeans of the stimulated Brillouin scattering (SBS) cell and the one ormore Brillouin amplifier.
 4. The system of claim 1, and also comprisingmeans for supplying the input beam of fundamental radiation.
 5. Thesystem of claim 4 wherein the means for supplying the input beam offundamental radiation is a laser.
 6. The system of claim 1, and alsoincluding a sub-system for deriving an input beam of radiation from abeam of primary radiation of frequency ω, wherein said sub-systemcomprises;an additional sample of non linear optical material forreceiving the beam of primary radiation and generating output radiationcomprising primary radiation, ω, and second harmonic radiation, 2ω,means for separating the beams of primary and second harmonic radiationoutput from the additional sample and means for increasing the intensityof radiation incident on the first sample, whereby the beam of secondharmonic radiation output from the sub-system is input to the system,such that that system may be used to generate at least two output beamsof fourth harmonic radiation, the two or more output beams of fourthharmonic radiation being coherently combined by means of the stimulatedBrillouin scattering (SBS) cell and the one or more Brillouin amplifier.7. The system of claim 6 comprising a first sample of non linear opticalmaterial and two further samples of non linear optical material suchthat three output beams of fourth harmonic radiation may be generated,the two or more output beams of fourth harmonic radiation beingcoherently combined by means of the stimulated Brillouin scattering(SBS) cell and the one or more Brillouin amplifier.
 8. The system ofclaim 7, and also comprising means for supplying primary radiation tothe sub-system.
 9. The system of claim 8 wherein the means for supplyingprimary radiation to the sub-system is a laser.
 10. The system of claim1 wherein the means for increasing the intensity of radiation incidenton any of the first, additional or further samples is system ofanamorphic prisms.
 11. The system of claim 1 wherein the means forincreasing the intensity of radiation incident on any of the first orfurther samples is a refracting telescope.
 12. The system of claim 1wherein the means for increasing the intensity of radiation incident onany of the first or further samples is system of reflecting telescopes.13. The system of claim 1 wherein the means for separating fundamentalradiation and higher harmonic radiation output from any of the first,additional or further samples are chromatic separators.
 14. The systemof claim 13 wherein the chromatic separators are any one of dichroicmirrors, prisms or polarisers.
 15. The system of claim 1, and alsoincluding a sub-system for deriving an input beam of radiation from abeam of primary radiation of frequency ω, wherein said sub-systemcomprises;an additional sample of non linear optical material forreceiving the beam of primary radiation and generating output radiationcomprising primary radiation, ω, and second harmonic radiation, 2ω,whereby the beam of radiation output from the sub-system is input to thesystem such that the system may be used to generate at least two outputbeams of third harmonic radiation, the two or more output beams of thirdharmonic radiation being coherently combined by means of the stimulatedBrillouin scattering (SBS) cell and the one or more Brillouin amplifier.16. The system of claim 15, comprising a first sample of non linearoptical material and two further samples of non linear optical material,such that three output beams of third harmonic radiation may begenerated, the three output beams of third harmonic radiation beingcoherently combined by means of the stimulated Brillouin scattering(SBS) cell and the one or more Brillouin amplifier.
 17. The system ofclaim 16 and also comprising means for providing primary radiation. 18.The system of claim 17 wherein the means for providing primary radiationis a laser.
 19. The system of claim 1 wherein any of the samples of nonlinear optical material are non linear optical crystals.
 20. The systemof claim 19 wherein any one of the non linear optical crystals, is anyone of a potassium titanyl phosphate (KTP) crystal, a potassiumdihydrogen phosphate (KDP) crystal, a deurated KDP (KD*P), caesiumdihydrogen arsenate (CDA) crystal, a deuterated CDA (CD*A), a betabarium borate (BBO) crystal or a lithium triborate (LBO) crystal. 21.The system claim 1 wherein the means for increasing the intensity ofradiation incident on any of the first, additional or further sampleshave a variable magnification such that the relative intensities of thebeams of higher order harmonic radiation output from the system may bevaried by varying the magnification.
 22. The system of claim 1, andfurther comprising optical delay lines included in the beams of higherorder radiation to ensure temporal overlap of the beams in the one ormore Brillouin amplifier.